Task5.2 Why the tunnel?

Am at a loss to explain the tunnel on this one. I saw a couple of previous posts, but no real explanation of how the tunnel allows this to work. Starting from this point of not understanding whatever is going on here, my logic would dictate that if a tunnel is necessary between R3 & R2 it would also be necessary between R2 & R1. I am able to ping group from R2 through R1 to receiver, so clearly no tunnel would be necessary to R1, but why?